According to the Mead Project, the physiological basis of behavior is the response to a stimuli effected by the body's muscles or glands. These effects are triggered by nervous impulses that are passed along the neurons from the receptor that first received the stimulus.
Physiological causes for behaviors generally involve the nervous system in some way. Most psychologists agree that, in physical terms, responses to external stimuli are created by receptors sending information to the brain and central nervous system, which then send return signals to the glands and muscles with instructions that are based on both automatic and learned responses.
This means that, in a well-ordered system, certain behavioral responses are generally predictable, such as pulling away from an extremely hot surface or shivering when cold. This also applies to emotional stimuli received from the environment such as displays of affection from the people around you or the threat of violence.
However, because the system relies on two-way communication between neurons that need to fire in the correct sequence, conditions and chemicals that inhibit these signals can interfere with this process. Wikipedia explains that behaviors that are unexpected can therefore be symptoms of other problems within the body, in particular with the various chemical neurotransmitters that facilitate signal transmission between neurons.